Compartimentation et dynamique des fonctions nucléaires

Publications de l’équipe

Année de publication : 2013

Nathanaël Hozé, Myriam Ruault, Carlo Amoruso, Angela Taddei, David Holcman (2013 Apr 10)

Spatial telomere organization and clustering in yeast Saccharomyces cerevisiae nucleus is generated by a random dynamics of aggregation-dissociation.

Molecular biology of the cell : 1791-800, S1-10 : DOI : 10.1091/mbc.E13-01-0031 En savoir plus
Résumé

Spatial and temporal behavior of chromosomes and their regulatory proteins is a key control mechanism in genomic function. This is exemplified by the clustering of the 32 budding yeast telomeres that form foci in which silencing factors concentrate. To uncover the determinants of telomere distribution, we compare live-cell imaging with a stochastic model of telomere dynamics that we developed. We show that random encounters alone are inadequate to produce the clustering observed in vivo. In contrast, telomere dynamics observed in vivo in both haploid and diploid cells follows a process of dissociation-aggregation. We determine the time that two telomeres spend in the same cluster for the telomere distribution observed in cells expressing different levels of the silencing factor Sir3 protein, limiting for telomere clustering. We conclude that telomere clusters, their dynamics, and their nuclear distribution result from random motion, aggregation, and dissociation of telomeric regions, specifically determined by the amount of Sir3.

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Peter Meister, Angela Taddei (2013 Jan 15)

Building silent compartments at the nuclear periphery: a recurrent theme.

Current opinion in genetics & development : 96-103 : DOI : 10.1016/j.gde.2012.12.001 En savoir plus
Résumé

In eukaryotes, the genetic material is stored in the nucleus, which is enclosed in a double lipid bilayer, the nuclear envelope (NE). It protects the genome from physical stress and separates it from the rest of the cell. On top of this physical function, growing evidence shows that the nuclear periphery contributes to the 3D organization of the genome. In turn, tridimensional organization of chromatin in the nuclear space influences genome expression. Here we review recent findings on the function of this physical barrier in gene repression and latest models on how silent subnuclear compartments at the NE are built in yeast as well as in the nematode C. elegans and mammalian cells; trying to draw parallels between the three systems.

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Année de publication : 2012

Angela Taddei, Susan M Gasser (2012 Sep 12)

Structure and function in the budding yeast nucleus.

Genetics : 107-29 : DOI : 10.1534/genetics.112.140608 En savoir plus
Résumé

Budding yeast, like other eukaryotes, carries its genetic information on chromosomes that are sequestered from other cellular constituents by a double membrane, which forms the nucleus. An elaborate molecular machinery forms large pores that span the double membrane and regulate the traffic of macromolecules into and out of the nucleus. In multicellular eukaryotes, an intermediate filament meshwork formed of lamin proteins bridges from pore to pore and helps the nucleus reform after mitosis. Yeast, however, lacks lamins, and the nuclear envelope is not disrupted during yeast mitosis. The mitotic spindle nucleates from the nucleoplasmic face of the spindle pole body, which is embedded in the nuclear envelope. Surprisingly, the kinetochores remain attached to short microtubules throughout interphase, influencing the position of centromeres in the interphase nucleus, and telomeres are found clustered in foci at the nuclear periphery. In addition to this chromosomal organization, the yeast nucleus is functionally compartmentalized to allow efficient gene expression, repression, RNA processing, genomic replication, and repair. The formation of functional subcompartments is achieved in the nucleus without intranuclear membranes and depends instead on sequence elements, protein-protein interactions, specific anchorage sites at the nuclear envelope or at pores, and long-range contacts between specific chromosomal loci, such as telomeres. Here we review the spatial organization of the budding yeast nucleus, the proteins involved in forming nuclear subcompartments, and evidence suggesting that the spatial organization of the nucleus is important for nuclear function.

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Frank R Neumann, Vincent Dion, Lutz R Gehlen, Monika Tsai-Pflugfelder, Roger Schmid, Angela Taddei, Susan M Gasser (2012 Feb 21)

Targeted INO80 enhances subnuclear chromatin movement and ectopic homologous recombination.

Genes & development : 369-83 : DOI : 10.1101/gad.176156.111 En savoir plus
Résumé

Chromatin in the interphase nucleus moves in a constrained random walk. Despite extensive study, the molecular causes of such movement and its impact on DNA-based reactions are unclear. Using high-precision live fluorescence microscopy in budding yeast, we quantified the movement of tagged chromosomal loci to which transcriptional activators or nucleosome remodeling complexes were targeted. We found that local binding of the transcriptional activator VP16, but not of the Gal4 acidic domain, enhances chromatin mobility. The increase in movement did not correlate strictly with RNA polymerase II (PolII) elongation, but could be phenocopied by targeting the INO80 remodeler to the locus. Enhanced chromatin mobility required Ino80’s ATPase activity. Consistently, the INO80-dependent remodeling of nucleosomes upon transcriptional activation of the endogenous PHO5 promoter enhanced chromatin movement locally. Finally, increased mobility at a double-strand break was also shown to depend in part on the INO80 complex. This correlated with increased rates of spontaneous gene conversion. We propose that local chromatin remodeling and nucleosome eviction increase large-scale chromatin movements by enhancing the flexibility of the chromatin fiber.

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Année de publication : 2011

Marion Dubarry, Isabelle Loïodice, Chunlong L Chen, Claude Thermes, Angela Taddei (2011 Jul 5)

Tight protein-DNA interactions favor gene silencing.

Genes & development : 1365-70 : DOI : 10.1101/gad.611011 En savoir plus
Résumé

The heterochromatin-like structure formed by the yeast silent information regulator complex (SIR) represses transcription at the silent mating type loci and telomeres. Here, we report that tight protein-DNA complexes induce ectopic recruitment of the SIR complex, promoting gene silencing and changes in subnuclear localization when cis-acting elements are nearby. Importantly, lack of the replication fork-associated helicase Rrm3 enhances this induced gene repression. Additionally, Sir3 and Sir4 are enriched genome-wide at natural replication pause sites, including tRNA genes. Consistently, inserting a tRNA gene promotes SIR-mediated silencing of a nearby gene. These results reveal that replication stress arising from tight DNA-protein interactions favors heterochromatin formation.

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Myriam Ruault, Arnaud De Meyer, Isabelle Loïodice, Angela Taddei (2011 Feb 9)

Clustering heterochromatin: Sir3 promotes telomere clustering independently of silencing in yeast.

The Journal of cell biology : 417-31 : DOI : 10.1083/jcb.201008007 En savoir plus
Résumé

A general feature of the nucleus is the organization of repetitive deoxyribonucleic acid sequences in clusters concentrating silencing factors. In budding yeast, we investigated how telomeres cluster in perinuclear foci associated with the silencing complex Sir2-Sir3-Sir4 and found that Sir3 is limiting for telomere clustering. Sir3 overexpression triggers the grouping of telomeric foci into larger foci that relocalize to the nuclear interior and correlate with more stable silencing in subtelomeric regions. Furthermore, we show that Sir3’s ability to mediate telomere clustering can be separated from its role in silencing. Indeed, nonacetylable Sir3, which is unable to spread into subtelomeric regions, can mediate telomere clustering independently of Sir2-Sir4 as long as it is targeted to telomeres by the Rap1 protein. Thus, arrays of Sir3 binding sites at telomeres appeared as the sole requirement to promote trans-interactions between telomeres. We propose that similar mechanisms involving proteins able to oligomerize account for long-range interactions that impact genomic functions in many organisms.

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Lutz R Gehlen, Shigeki Nagai, Kenji Shimada, Peter Meister, Angela Taddei, Susan M Gasser (2011 Jan 4)

Nuclear geometry and rapid mitosis ensure asymmetric episome segregation in yeast.

Current biology : CB : 25-33 : DOI : 10.1016/j.cub.2010.12.016 En savoir plus
Résumé

Asymmetric cell division drives the generation of differentiated cells and maintenance of stem cells. In budding yeast, autonomously replicating sequence (ARS) plasmids lacking centromere elements are asymmetrically segregated into the mother cell, where they are thought to contribute to cellular senescence. This phenomenon has been proposed to result from the active retention of plasmids through an interaction with nuclear pores.

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Année de publication : 2010

Angela Taddei, Heiko Schober, Susan M Gasser (2010 Jun 18)

The budding yeast nucleus.

Cold Spring Harbor perspectives in biology : a000612 : DOI : 10.1101/cshperspect.a000612 En savoir plus
Résumé

The budding yeast nucleus, like those of other eukaryotic species, is highly organized with respect to both chromosomal sequences and enzymatic activities. At the nuclear periphery interactions of nuclear pores with chromatin, mRNA, and transport factors promote efficient gene expression, whereas centromeres, telomeres, and silent chromatin are clustered and anchored away from pores. Internal nuclear organization appears to be function-dependent, reflecting localized sites for tRNA transcription, rDNA transcription, ribosome assembly, and DNA repair. Recent advances have identified new proteins involved in the positioning of chromatin and have allowed testing of the functional role of higher-order chromatin organization. The unequal distribution of silent information regulatory factors and histone modifying enzymes, which arises in part from the juxtaposition of telomeric repeats, has been shown to influence chromatin-mediated transcriptional repression. Other localization events suppress unwanted recombination. These findings highlight the contribution budding yeast genetics and cytology have made to dissecting the functional role of nuclear structure.

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Takahito Yoshida, Kenji Shimada, Yukako Oma, Véronique Kalck, Kazumi Akimura, Angela Taddei, Hitoshi Iwahashi, Kazuto Kugou, Kunihiro Ohta, Susan M Gasser, Masahiko Harata (2010 Apr 27)

Actin-related protein Arp6 influences H2A.Z-dependent and -independent gene expression and links ribosomal protein genes to nuclear pores.

PLoS genetics : e1000910 : DOI : 10.1371/journal.pgen.1000910 En savoir plus
Résumé

Actin-related proteins are ubiquitous components of chromatin remodelers and are conserved from yeast to man. We have examined the role of the budding yeast actin-related protein Arp6 in gene expression, both as a component of the SWR1 complex (SWR-C) and in its absence. We mapped Arp6 binding sites along four yeast chromosomes using chromatin immunoprecipitation from wild-type and swr1 deleted (swr1Delta) cells. We find that a majority of Arp6 binding sites coincide with binding sites of Swr1, the catalytic subunit of SWR-C, and with the histone H2A variant Htz1 (H2A.Z) deposited by SWR-C. However, Arp6 binding detected at centromeres, the promoters of ribosomal protein (RP) genes, and some telomeres is independent of Swr1 and Htz1 deposition. Given that RP genes and telomeres both show association with the nuclear periphery, we monitored the ability of Arp6 to mediate the localization of chromatin to nuclear pores. Arp6 binding is sufficient to shift a randomly positioned locus to nuclear periphery, even in a swr1Delta strain. Arp6 is also necessary for the pore association of its targeted RP promoters possibly through cell cycle-dependent factors. Loss of Arp6, but not Htz1, leads to an up-regulation of these RP genes. In contrast, the pore-association of GAL1 correlates with Htz1 deposition, and loss of Arp6 reduces both GAL1 activation and peripheral localization. We conclude that Arp6 functions both together with the nucleosome remodeler Swr1 and also without it, to mediate Htz1-dependent and Htz1-independent binding of chromatin domains to nuclear pores. This association is shown to have modulating effects on gene expression.

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Hani Ebrahimi, E Douglas Robertson, Angela Taddei, Susan M Gasser, Anne D Donaldson, Shin-ichiro Hiraga (2010 Mar 4)

Early initiation of a replication origin tethered at the nuclear periphery.

Journal of cell science : 1015-9 : DOI : 10.1242/jcs.060392 En savoir plus
Résumé

Peripheral nuclear localization of chromosomal loci correlates with late replication in yeast and metazoan cells. To test whether peripheral positioning can impose late replication, we examined whether artificial tethering of an early-initiating replication origin to the nuclear periphery delays its replication in budding yeast. We tested the effects of three different peripheral tethering constructs on the time of replication of the early replication origin ARS607. Using the dense-isotope transfer method to assess replication time, we found that ARS607 still replicates early when tethered to the nuclear periphery using the Yif1 protein or a fragment of Sir4, whereas tethering using a Yku80 construct produces only a very slight replication delay. Single-cell microscopic analysis revealed no correlation between peripheral positioning of ARS607 in individual cells and delayed replication. Overall, our results demonstrate that a replication origin can initiate replication early in S phase, even if artificially relocated to the nuclear periphery.

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Année de publication : 2009

Angela Taddei, Griet Van Houwe, Shigeki Nagai, Ionas Erb, Erik van Nimwegen, Susan M Gasser (2009 Jan 31)

The functional importance of telomere clustering: global changes in gene expression result from SIR factor dispersion.

Genome research : 611-25 : DOI : 10.1101/gr.083881.108 En savoir plus
Résumé

Budding yeast telomeres and cryptic mating-type loci are enriched at the nuclear envelope, forming foci that sequester silent information regulators (SIR factors), much as heterochromatic chromocenters in higher eukaryotes sequester HP1. Here we examine the impact of such subcompartments for regulating transcription genome-wide. We show that the efficiency of subtelomeric reporter gene repression depends not only on the strength of SIR factor recruitment by cis-acting elements, but also on the accumulation of SIRs in such perinuclear foci. To monitor the effects of disrupting this subnuclear compartment, we performed microarray analyses under conditions that eliminate telomere anchoring, while preserving SIR complex integrity. We found 60 genes reproducibly misregulated. Among those with increased expression, 22% were within 20 kb of a telomere, confirming that the nuclear envelope (NE) association of telomeres helps repress natural subtelomeric genes. In contrast, loci that were down-regulated were distributed over all chromosomes. Half of this ectopic repression was SIR complex dependent. We conclude that released SIR factors can promiscuously repress transcription at nontelomeric genes despite the presence of « anti-silencing » mechanisms. Bioinformatic analysis revealed that promoters bearing the PAC (RNA Polymerase A and C promoters) or Abf1 binding consenses are consistently down-regulated by mislocalization of SIR factors. Thus, the normal telomeric sequestration of SIRs both favors subtelomeric repression and prevents promiscuous effects at a distinct subset of promoters. This demonstrates that patterns of gene expression can be regulated by changing the spatial distribution of repetitive DNA sequences that bind repressive factors.

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Année de publication : 2008

Myriam Ruault, Marion Dubarry, Angela Taddei (2008 Sep 30)

Re-positioning genes to the nuclear envelope in mammalian cells: impact on transcription.

Trends in genetics : TIG : 574-81 : DOI : 10.1016/j.tig.2008.08.008 En savoir plus
Résumé

The spatial organization of the genome within the nucleus is thought to contribute to genome functions. A key component of the nuclear architecture is the nuclear envelope, which is often associated with inactive chromatin. Studies in budding yeast indicate that nuclear position can directly affect gene function. However, the causal relationship between gene position and gene activity in mammalian cells has been more elusive. Several groups recently addressed this issue by tethering genes to the inner nuclear membrane. Their studies show that the nuclear periphery is not refractory to gene transcription, but can modulate the activity of certain genes. The 3D organization of the genome might, thus, provide an additional level of regulation necessary for fine-tuning gene expression.

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Année de publication : 2007

Angela Taddei (2007 May 1)

Active genes at the nuclear pore complex.

Current opinion in cell biology : 305-10 En savoir plus
Résumé

The nucleus is spatially and functionally organized and its architecture is now seen as a key contributor to genome functions. A central component of this architecture is the nuclear envelope, which is studded with nuclear pore complexes that serve as gateways for communication between the nucleoplasm and cytoplasm. Although the nuclear periphery has traditionally been described as a repressive compartment and repository for gene-poor chromosome regions, several recent studies in yeast have demonstrated that repressive and activating domains can both be positioned at the periphery of the nucleus. Moreover, association with the nuclear envelope favors the expression of particular genes, demonstrating that nuclear organization can play an active role in gene regulation.

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Peter Meister, Angela Taddei, Aaron Ponti, Giuseppe Baldacci, Susan M Gasser (2007 Feb 17)

Replication foci dynamics: replication patterns are modulated by S-phase checkpoint kinases in fission yeast.

The EMBO journal : 1315-26 En savoir plus
Résumé

Although the molecular enzymology of DNA replication is well characterised, how and why it occurs in discrete nuclear foci is unclear. Using fission yeast, we show that replication takes place in a limited number of replication foci, whose distribution changes with progression through S phase. These sites define replication factories which contain on average 14 replication forks. We show for the first time that entire foci are mobile, able both to fuse and re-segregate. These foci form distinguishable patterns during S phase, whose succession is reproducible, defining early-, mid- and late-S phase. In wild-type cells, this same temporal sequence can be detected in the presence of hydroxyurea (HU), despite the reduced rate of replication. In cells lacking the intra-S checkpoint kinase Cds1, replication factories dismantle on HU. Intriguingly, even in the absence of DNA damage, the replication foci in cds1 cells assume a novel distribution that is not present in wild-type cells, arguing that Cds1 kinase activity contributes to the spatio-temporal organisation of replication during normal cell growth.

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Année de publication : 2006

Peter Meister, Angela Taddei, Susan M Gasser (2006 Jul 4)

In and out of the replication factory.

Cell : 1233-5 En savoir plus
Résumé

In this issue of Cell, use live-fluorescence microscopy to monitor individual genomic loci as they replicate in budding yeast. They confirm that DNA is recruited to replication factories and show that sister replication forks initiated from the same origin are held together within a single replication factory.

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